When trying to decide on the best type of bike, think about these questions:
Who do you ride with?
What do they ride?
What have you had in the past that you liked?
What have you had in the past that you disliked?
What would you like to do with your new bike?
If you are buying a bike to ride with a group of friends, buy something similar to what they ride. You will not be able to keep up with road bikes if you are on a mountain bike or cruiser. And a road bike cannot go on the dirt or the sand.
If you enjoyed a three speed as a child, you may find this a fun bike again. If you disliked the road bike you bought a few years ago, perhaps a mountain bike would be better.
Road bicycles are designed to be ridden fast on smooth pavement. They have smooth, skinny tires and "drop" handlebars, and can be used for on-road racing. They are usually lighter than other types of bicycles. They can be ridden on paved trails, but most people find them uncomfortable and unstable on unpaved trails. Most road bikes are not capable of carrying heavy loads.
Cyclocross bicycles are a special type of road bike designed to be ridden on mixed surfaces (combination of pavement, unpaved trails, gravel, grass). They have a drop handlebar like regular road bikes, but the tires are a littler wider for more off-road traction, and they have a different style of brake that helps to prevent mud buildup in the frame. Cyclocross bikes make good commuter bikes, because of the versatility, durability, and in most cases, the ability to carry light loads. Cyclocross bicycles are sometimes called 'cross bikes or cx bikes for short.
Touring bicycles are another special type of road bike. They are designed to be ridden on pavement, but are more durable for use on self-supported long-distance riding. They have all of the necessary mounting bolts for cargo racks and fenders, and although they still have a drop handlebar, they usually have a more relaxed frame design so that the rider is more upright, for more comfort when riding long distances for multiple days at a time. They have a lower gear range compared to regular road bikes, to allow for carrying heavy loads up steep hills. They also make good commuter bicycles, because of their durability and ability to carry heavy loads.
Triathlon/Time Trial Bicycles are road bikes with a special design that maximizes their aerodynamic properties. The handlebars are also a special aerodynamic design that allows you to crouch forward while riding, to minimize the wind resistance against your body. Triathlon/time trial races usually have staggered starts, where each racer starts on his/her own; these bicycles are usually not allowed to be used in mass-start races.
Flat-Bar Road Bicycles have most of the advantages of regular road bikes--lightweight frames and relatively narrow tires for efficiency on pavement--with a flat or upright handlebar. These bikes are designed for people who want a light, high-performance bike, but don't like the drop-handlebar riding position of a regular road bike. These bicycles are sometimes called fitness bikes or performance hybrid bikes. Most of them can accept somewhat wider tires, to make them suitable for use on unpaved trails. They usually have the ability to mount cargo racks and fenders, which make them good commuter bikes.
Track/Fixed-Gear Bicycles or fixies are designed to be ridden on a velodrome, which is a banked oval track specifically for bicycle racing. Some commuters prefer track bikes, however, due to their simple design, which makes them easy to maintain. They have a single gear that does not provide the ability to coast, so if the bike is moving, your feet must be pedaling. For even more simplicity, some riders prefer to not have brakes, since the fixed-gear mechanism can act as a brake. Most track bikes have drop handlebars, but some riders outfit theirs with flat or upright handlebars.
Mountain Bicycles are design for riding rough off-road trails. They have flat or upright handlebars, and a very low gear range for pedaling up steep trails. Most mountain bikes have some type of shock absorbers or suspension. Mountain bikes with front suspension only are called hardtails; mountain bikes with both front and rear suspension are called full-suspension bikes or duallies. Mountain bikes with no suspension are called rigid. Mountain bikes can be outfitted for use as touring or commuting bikes, although they would not be as light or efficient as traditional touring or commuting bikes.
Hybrid Bicycles were originally conceived to provide the advantages of both road bikes and mountain bikes. Their large, padded seats and upright handlebars provide a comfortable riding position, and are best for casual riding around the neighborhood or bike paths, short-distance commuting, and errands around town. They can be ridden on paved roads, but are not as lightweight or efficient as road bikes. They are ideal for paved or unpaved bike trails, but are not appropriate for rough off-road mountain bike trails. The tires are usually a medium-width with a semi-smooth tread, to provide a fairly smooth ride on pavement, but enough grip and cushion on unpaved trails. Most hybrid bikes have front suspension to smooth out small bumps, but some are fully rigid. Hybrid bikes used to also be referred to as cross bikes, but that term is not used any more in order to avoid confusion with cyclocross bikes (see above).
Performance Hybrid Bicycles are oriented towards riders who want the multi-surface versatility of a hybrid bike, but want a little more aggressive style and riding position. They have a flat or upright handlebar, although not as upright as regular hybrid bikes; they usually have a smaller, more performance-oriented seat, rather than a large comfort seat. Some have front suspension; some do not. Performance hybrids make good commuter bikes, and are also good for touring on unpaved trails. There is much overlap between the performance hybrid category and flat-bar road bikes (see above); the terms are almost used interchangeably.
Cruiser Bicycles are similar to hybrid bikes, in that they are designed for casual riding, and have a very comfortable, upright riding position, and a large, comfortable seat. Cruisers usually have wide "balloon" tires, and handlebars that are even more upright, and in some cases, swept back compared to hybrid bikes. Most cruiser bikes are single-speed or 3-speed, and have the old-fashioned coaster brake (where you pedal backwards to stop). They can be used for short-distance commuting and errands, as long as your route is fairly flat. Some cruiser bike manufacturers make a wide array of colorful models available, to suit the fashion tastes of any bike afficionado.
Flat-Foot Comfort Bikes are a sub-category of cruiser bikes. They have an elongated frame design that pushes the pedals a few inches forward of the seat. This allows you to ride with the seat low enough so that you can place your feet flat on the ground when you are stopped, but you still get the full extension of your legs while pedaling. All Electra Bicycle Company bikes have the flat-foot technology; some other manufacturers have created their own flat-foot designs.
The term "city bike" doesn't really refer to a specific category of bikes; it's more of a general descriptive term. They might also be called "commuter" or "urban" bikes, although any of the bikes listed on this page can be used quite well for riding and commuting in a city. However, there is a certain type of bike that most people have in mind when they use the term "city bike." This bike has characteristics of both a hybrid bike and a cruiser bike--usually the upright riding position of a cruiser, but the wheel size of a hybrid bike.
A city bike might also have some or all of these features that make it more amenable to riding in regular clothes, as opposed to cycling-specific clothing:
Skirt guard on rear wheel
A city bike might also have an internally-geared rear hub for ease of use and maintenance, and a built-in generator and lights for safety when riding after dark.
These bikes are also sometimes called "Dutch bikes," because of their resemblance to the everyday bikes used in Amsterdam and other bike-friendly European cities.
BMX Bikes are popular with kids because of their small size, but they are used by adults and kids alike for various styles of trick and stunt riding.
Folding Bicycles are ideal for those who need to travel with their bike, want a bike to keep on their boat or plane, or who live in small apartments and don't have a lot of storage space. They're also good for commuters who need to take their bike on a bus or train for part of their commute, or who don't have a safe place to park their bike at work. Most folding bikes have smaller wheels, which makes the bike a little less efficient and trickier to handle than a standard bike, but most folding bike fans feel the benefits outweigh the disadvantages.
Recumbent Bicycles have a long, low design and a full-size seat with a backrest. Recumbents are available in two-wheel and three-wheel designs. Many recumbent riders feel that they are the most comfortable option available for bicycling. However, they are more difficult to pedal up hills, and they can be a challenge to carry from one place to another in a motor vehicle.
Tandem Bicycles are "bicycles built for two." They come in all styles, from cruiser tandems and hybrid tandems for the bike path or boardwalk, off-road mountain bike tandems, and high-performance road racing tandems.
Adult Trikes are ideal for older folks who still want to get around under their own power, or those with balance issues or other special needs. They are also popular in environmentally-consious industrial/warehouse applications.